1. Field of the Invention
Aspects of the present invention relate to an optical disc apparatus and a focusing method for the optical disc apparatus, and more particularly, to an optical disc apparatus and a focusing method for the optical disc apparatus which efficiently prevent a focusing start error of an optical pickup when using a ring-shaped lens to handle multiple wavelengths and allow the optical pickup to start focusing correctly.
2. Description of the Related Art
Generally, an optical disc apparatus emits a laser beam to an optical disc to write data onto the optical disc, or reads a laser beam reflected from the optical disc containing data recorded onto the optical disc.
The laser wavelength and the numerical aperture (NA) of the optical disc apparatus are adjusted depending on the storage capacity of the optical disc apparatus. An infrared laser beam having a wavelength of about 70 nanometers (nm) and an objective lens having a numerical aperture of about 0.4 are used to read data from and write data onto compact discs (CDs). A red laser beam having a wavelength in the range of 635-650 nm and an objective lens having a numerical aperture of about 0.6 are used to read data from and write data onto digital versatile discs (DVDs).
A blue laser beam having a wavelength of about 405 nm, which is a shorter wavelength than the wavelength of the laser beam used to read data from and write data onto DVDs, is used to read data from and write data onto Blu-ray Discs (BD).
As the wavelength of the laser beam used to write data onto and read data from an optical disc becomes smaller, the track pitch of the optical disc may be correspondingly reduced so that a larger amount of data may be stored onto an optical disc of the same size.
DVD writer/reader units are generally designed to be compatible with CD writer/reader units, so that the DVD writer/reader units can write and/or read data to and/or from not only DVDs, but also CDs. DVD writer/reader units which handle multiple wavelengths are divided into two types: DVD writer/reader units which use a plurality of lenses corresponding to multiple wavelengths, and DVD writer/reader units which use only a single lens designed to handle multiple wavelengths. The DVD writer/reader units which use only a single lens are easy to design and cost less to manufacture than the DVD writer/reader units which use multiple lenses, and are therefore more commonly used today.
When writing and/or reading data to and/or from an optical disc, it is necessary to accurately focus a laser beam onto a recording layer of the optical disc. To accurately focus a laser beam onto a recording layer of the optical disc, a plurality of divided signals is generated from a laser beam reflected by the optical disc, and a focus error signal is in turn generated from the plurality of divided signals. An optical disc apparatus generally includes an objective lens, an actuator used to control the focus of the laser beam which writes and reads data to and from an optical disc, and a focus controller used to control the actuator. The focus controller in the optical disc apparatus typically controls the actuator using a focus error signal which enables the focus controller to keep the actuator continuously positioned so that the actuator accurately follows a recording layer of an optical disc.
When a user inserts an optical disc into an optical disc apparatus and issues a write or read command, the optical disc apparatus performs a focus search operation to obtain a focus error signal. The focus error signal enables the focus controller to commence a focus control operation, using the focus error signal to ensure that the focus of the laser beam continuously follows the recording layer of the optical disc. Specifically, the focus search operation detects a zero crossing point (ZCP) of the focus error signal while moving the objective lens vertically above the surface of the optical disc. Once the focus search operation detects the ZCP, the focus control operation is started based on the detected ZCP.
As illustrated in FIG. 7, when multiple wavelengths are handled using a single lens 22 to write and/or read data to and/or from various types of optical discs, the single lens 22 is generally designed to be a ring-shaped lens having a surface with multiple diffraction rings 22a and 22b corresponding respectively to the multiple wavelengths. However, when this ring-shaped lens 22 having a surface with multiple diffraction rings 22a and 22b is used during the focus search operation, the ring-shaped lens causes an abnormal parasitic signal, as shown in FIG. 1.
The parasitic signal or external noise may cause the focus search operation to incorrectly detect the parasitic signal or external noise as the ZCP of the focus error signal, resulting in incorrect focusing of the objective lens on the recording layer, or resulting in failure to start the focusing control operation.
The conventional focusing method, in which the focusing control operation is based on the amplitude of the focus error signal, cannot prevent the incorrect focusing of the objective lens onto the recording layer of the optical disc, or prevent a failure of the focusing control operation from starting, when the amplitude of the parasitic signal or noise is high enough that the focusing control operation incorrectly detects the parasitic signal or external noise as the ZCP.